The 70 mph National Speed Limit was introduced as a temporary measure in December 1965. It is often blamed on Barbara Castle, but at the time the Minister of Transport was Tom Fraser. The reason given was a spate of serious accidents in foggy conditions, but it is often claimed that the MoT had been alarmed by AC Cars testing their latest Cobra on the M1 at speeds up to 180 mph. It was confirmed as a permanent limit in 1967, by which time Barbara Castle (a non-driver) had become Minister of Transport.There was surprisingly little debate at the time: the fact that the average family car of the time could only just exceed 70 mph perhaps had something to do with this. It should be noted that this limit applied to all previously "derestricted" roads, not only motorways. All rural roads in the Isle of Man (including most of the famous TT course) remain genuinely derestricted, as a matter of interest.